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The Supreme Court reaffirmed on Monday the right of corporations to make independent political expenditures, summarily overturning a 100-year-old Montana state law that barred companies from such political activity.

The justices ruled in an unsigned opinion that Montana's law was in conflict with the court's 2010 Citizens United decision, which shifted the campaign finance landscape, opening the door to the huge political expenditures that have been shaping this year's presidential race. The decision was 5-4, split along ideological lines.

Despite the Citizens United decision, the Montana Supreme Court had refused to strike down the state's ban on election spending by corporations. Its judges cited Montana's history of "copper kings" who bribed legislators. Advocates of campaign finance reform had hoped that the current wave of election-related spending would help make their case for the need to reconsider Citizens United.

Advocates for stricter campaign finance regulations said they will continue their fight for tougher laws in Congress and the courts.

Montana Attorney General Steve Bullock says the U.S. Supreme Court decision to strike down Montana's restrictions on corporate political spending was politically motivated. Gov. Brian Schweitzer was even blunter, saying the Supreme Court is now endorsing "dirty, secret, corporate, foreign money." The high court overturned the state campaign finance restrictions in a case that piggybacked the 2010 Citizens United decision that allowed corporations to spend freely to influence elections.